1. Field of the Invention
This document relates to an organic light emitting display. More particularly, the document relates to an organic light emitting display which is capable of sensing electrical characteristics of a driving element.
2. Discussion of the Related Art
An active matrix-type organic light emitting display comprises a self-emissive organic light emitting diode (hereinafter, referred to as “OLED”), and offers advantages such as fast response speed, high light emission efficiency, high luminance, and wide viewing angle.
An OLED, which is a self-emissive element, comprises an anode, a cathode, and organic compound layers HIL, HTL, EML, ETL, and EIL formed between the anode and the cathode. The organic compound layers comprise a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode and the cathode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML to form excitons. As a result, the emission layer EML generates visible light.
In an organic light emitting display, pixels each including an OLED are arranged in a matrix form, and the luminance of the pixels is controlled according to the grayscale of video data. Each pixel comprises a driving element, i.e., driving TFT (thin film transistor), that controls the driving current flowing through the OLED in response to a voltage Vgs applied between its gate electrode and source electrode. Electrical characteristics of the driving TFT, such as threshold voltage, mobility, etc, may be deteriorated with the passage of driving time, causing variations from pixel to pixel. These variations in the electrical characteristics of the driving TFT between the pixels make difference in the luminance of the same video data between the pixels. This makes it difficult to realize a desired image.
An internal compensation method and an external compensation method are known to compensate for variations in electrical characteristics of a driving TFT. In the internal compensation method, variations in the threshold voltage of driving TFTs are automatically compensated for within a pixel circuit. The configuration of the pixel circuit is very complicated because the driving current flowing through the OLED has to be determined regardless of the threshold voltage of the driving TFTs for the sake of internal compensation. Moreover, the internal compensation method is inappropriate to compensate for mobility variations between the driving TFTs.
In the external compensation method, variations in electrical characteristics are compensated for by measuring sensed voltages corresponding to the electrical characteristics (threshold voltage and mobility) of the driving TFTs and modulating video data by an external circuit based on these sensed voltages. In recent years, research on the external compensation method is actively underway.
In the conventional external compensation method, a data driving circuit receives a sensed voltage from each pixel through a sensing line, converts the sensed voltage into a digital sensed value, and then transmits the sensed value to a timing controller. The timing controller modulates digital video data based on the digital sensed value and compensates for variations in electrical characteristics of a driving TFT.
As the driving TFT is a current element, its electrical characteristics are represented by the amount of current Ids flowing between a drain and a source in response to a given gate-source voltage Vgs. By the way, the data driving circuit of the conventional external compensation method senses a voltage corresponding to the current Ids, rather than sensing the current Ids flowing through the driving TFT, in order to sense the electrical characteristics of the driving TFT.
For instance, in the external compensation method disclosed in Korean Patent Nos. 10 2013-0134256, filed Dec. 10, 2013 and 10-2013-0149395, filed Dec. 3, 2013, by the present applicant, LG Display Co., Ltd., the driving TFT is operated in a source follower manner, and then a voltage (driving TFT's source voltage) stored in the line capacitor (parasitic capacitor) of the sensing line is sensed by the data driving circuit. In this external compensation method, the source voltage is sensed when the source electrode potential of the driving TFT DT operating in the source follower manner reaches a saturation state (i.e., the current Ids of the driving TFT DT becomes zero), in order to compensate for variations in the threshold voltage of the driving TFT. Also, in this external compensation method, a linear voltage is sensed before the source electrode potential of the driving TFT DT operating in the source follower manner reaches a saturation state, in order to compensate for variations in the mobility of the driving TFT.
The conventional external compensation method has the following problems.
First, the source voltage is sensed after the current flowing through the driving TFT is changed into the source voltage and stored by using the parasitic capacitor of the sensing line. In this case, the parasitic capacitance of the sensing line is rather large, and moreover the amount of parasitic capacitance may change with the display load of the display panel. Because parasitic capacitance is not kept at a constant level but changes due to a variety of environmental factors, it cannot be calibrated. Any change in the amount of parasitic capacitance where current is stored makes it difficult to obtain an accurate sensed value.
Second, it takes quite a long time to obtain a sensed value, for example, until the source voltage of the driving TFT is saturated, because the conventional external compensation method employs voltage sensing. Especially, if the parasitic capacitance of the sensing line is large, it takes much time to draw enough current to meet a voltage level at which sensing is enabled. This problem becomes more serious in the case of low-grayscale sensing than in the case of high-grayscale sensing, as shown in FIG. 1.